1. Field of the Invention
The present invention relates generally to the field of digital wireless communications. Specifically, the present invention relates to architectures of a digitally controlled wideband radio frequency (RF) modulator and synthesizer system. More specifically, the present invention relates to frequency modulation architectures that can operate in different modulation standards and different frequency bands by using adaptive digital signal processing techniques.
2. Description of the Related Art
There are many challenging implementation issues associated with the direct modulation of a VCO with an analog waveform consistent with the instantaneous frequency specification. The main problem is due to frequency instability, drift and phase noise in the VCO. A typical approach to solve this problem is to directly modulate a phase-locked loop (PLL) frequency synthesizer rather than directly modulating a VCO. One example of direct phase modulation is to use a fractional-N frequency synthesizer 100 as illustrated in FIG. 1. But the bandwith of the fractional-N frequency synthesizer 100 is strongly limited by its phase-looked loop formed by a divider 104, a phase detector 103, a loop filter 102 and a VCO 101. One significant drawback of the Fractional-N frequency synthesizer is that the modulation bandwidth must be less than the PLL bandwidth. This constraint imposes a severe conflict between achieving high data rates and good noise performance. The high data rates require a wide PLL bandwidth, but low output noise requires a low PLL bandwidth in order to properly attenuate the sigma-delta quantization noise.
In particular, the article “A 27-mW CMOS Fractional-N Synthesizer Using Digital Compensation for 2.5 Mb/s GFSK Modulation” by Michael H. Perrott and etc., IEEE Journal of Solid-State Circuits, Vol. 32, No. 12, December 1997, pages 2048 to 2060, describes a wideband fractional-N synthesizer. It requires a compensated transmit filter that is difficult to implement, and an accurate loop filter to mitigate the mismatch problem. However, this approach is not efficient for the implementation of a wide band frequency modulator or synthesizer at low cost.
There is thus a great need for a frequency modulator or a frequency synthesizer architecture that can allow the modulation bandwidth to exceed the PLL bandwidth, while keeping low phase noise and eliminating frequency drift.